The regulatory change with the biggest impact on nuclear occurred before TMI. It was the Calvert Cliffs decision, a SCOTUS case that nuclear power plants were not exempt from the requirements of the National Environmental Policy Act.
While this had the largest short term impact, I would argue the larger cultural, organisational, regulatory approches had the more fundamental long-term effect.
https://en.wikipedia.org/wiki/Calvert_Cliffs_Coordinating_Co...